The amount or degree of coupling in a hydraulic friction clutch is typically adjusted by adjusting the amount of working fluid supplied from a supply chamber to a working chamber, which contains a drive disk and the driven component. A hydraulic friction clutch of this type is described in a German patent publication, DE 34 24 385 C2.
The advantage of such a hydraulic friction clutch is that moved valve parts for opening or closing the fluid connection between the working chamber and the supply chamber become superfluous, so that the life of such clutches is longer than in other types of clutches. In the known type, viscose working fluid is circulated to protect the clutch against possible overheating. To circulate, the feed duct for the working fluid is designed so that the fluid connection orifice or port located on the supply chamber side is arranged at a greater radial distance from the clutch's rotation axis than the level of the fluid ring formed in the supply chamber during the rotation of the driven component. By virtue of this design, when the supply chamber is pressurized with a pneumatic medium, it is possible for fluid to overflow from the supply chamber into the working chamber. The return flow to the supply chamber requires a pump, which pumps fluid in a known way through a return duct commencing in the radial outer region of the-working chamber. The chosen design ensures that, when the supply chamber is pressureless, working fluid cannot overflow from the working chamber to the supply chamber through the connecting duct. Working fluid from the working chamber is designed to return solely through the return duct.
Even though such a hydraulic friction clutch has advantages, its inflow and outflow arrangement has disadvantages. For instance, the exact quantity of fluid remaining in the working space is difficult to regulate. The pumping system arranged in the working space conveys a specific quantity per unit time back into the supply chamber as a function of relative rotational speed (the difference between the driving speed and the fan speed being equal to the slip) between the primary disk and the housing, of the viscosity of the working fluid, of the fan speed, and of the fluid quantity present in the working chamber. Thus, a specific state can occur only when the fluid quantity flowing in and out is at an equilibrium. Changes in the boundary conditions, such as changes in the driving speed, fan torque changes or the like, disturb this equilibrium and can delay the established fan speed change. Another disadvantage is that it is difficult to uniformly distribute fluid in the working chamber, upstream and downstream of the drive disk, because fluid first distributes radially outwardly from the inflow.
In this respect, a European patent publication, EP-A 0 005 927, proposes forcing fluid via pressure-loaded pistons into the working gaps of a hydraulic friction clutch having a T-shaped drive disk. A disadvantage of this arrangement is that the displacement pistons loaded with compressed air are located outside the diameter or periphery of the drive disk; this considerably increases the radial dimensions of the clutch, increasing the moment of inertia. Furthermore, controlling the clutch via fluid, whether gaseous or liquid, in this manner has other disadvantages.
There is a need for a hydraulic clutch that can rapidly and simply regulate the degree or amount of flow to and from the working chamber. The present invention meets this need.